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                Java Reflection Tutorial
              
            
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        <span itemprop="articleBody"><h4 id="Overview">Overview</h4><p><em>Reflection</em> is commonly used by programs which require the ability to examine or modify the runtime behavior of applications running in the Java virtual machine. This is a relatively advanced feature and should be used only by developers who have a strong grasp of the fundamentals of the language. With that caveat in mind, reflection is a powerful technique and can enable application to perform operations which would otherwise be impossible.</p>
<h4 id="Drawbacks_of_Reflection">Drawbacks of Reflection</h4><p><em>Reflection</em> is powerful, but should not be used indiscriminatey. If it is possible to perform an operation without using reflection, then it is preferable to avoid using it. The following concerns should be kept in mind when accessing code via reflection.</p>
<ul>
<li>Performance Overhead<br>  Because reflection involves types that are dynamically resolved, certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code which are called frequently in performance-sensitive applications.</li>
<li>Security Restrictions<br>  Reflection requires a runtime permission which may not be present when running under a security manager. This is an important consideration for code which has to run in a restricted security context, such as in an Applet.</li>
<li>Exposure of Internals<br>  Since reflection allows code to perform operations that would be illegal in non-reflective code, such as accessing private fields and methods, the use of reflection can result in unexpected side-effects, which may render code dysfunctional and may destroy portability. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.</li>
</ul>
<h4 id="Simple_Sample">Simple Sample</h4><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br><span class="line">100</span><br><span class="line">101</span><br><span class="line">102</span><br><span class="line">103</span><br><span class="line">104</span><br><span class="line">105</span><br><span class="line">106</span><br><span class="line">107</span><br><span class="line">108</span><br><span class="line">109</span><br><span class="line">110</span><br><span class="line">111</span><br><span class="line">112</span><br><span class="line">113</span><br><span class="line">114</span><br><span class="line">115</span><br><span class="line">116</span><br><span class="line">117</span><br><span class="line">118</span><br><span class="line">119</span><br><span class="line">120</span><br><span class="line">121</span><br><span class="line">122</span><br><span class="line">123</span><br><span class="line">124</span><br><span class="line">125</span><br><span class="line">126</span><br><span class="line">127</span><br><span class="line">128</span><br><span class="line">129</span><br><span class="line">130</span><br><span class="line">131</span><br><span class="line">132</span><br><span class="line">133</span><br><span class="line">134</span><br><span class="line">135</span><br><span class="line">136</span><br><span class="line">137</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line"><span class="keyword">import</span> java.lang.annotation.Inherited;</span><br><span class="line"><span class="keyword">import</span> java.lang.annotation.Retention;</span><br><span class="line"><span class="keyword">import</span> java.lang.annotation.RetentionPolicy;</span><br><span class="line"><span class="keyword">import</span> java.lang.reflect.Constructor;</span><br><span class="line"><span class="keyword">import</span> java.lang.reflect.InvocationTargetException;</span><br><span class="line"><span class="keyword">import</span> java.lang.reflect.Method;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">interface</span> <span class="title">Pluginable</span> </span>&#123;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">interface</span> <span class="title">Reflection</span> </span>&#123;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="annotation">@Inherited</span></span><br><span class="line"><span class="annotation">@Retention</span>(RetentionPolicy.RUNTIME)</span><br><span class="line"><span class="annotation">@interface</span> MyAnnotation &#123;</span><br><span class="line">    <span class="function">String <span class="title">value</span><span class="params">()</span> <span class="keyword">default</span> "defaultValue"</span>;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">boolean</span> <span class="title">isDebugMode</span><span class="params">()</span> <span class="keyword">default</span> <span class="keyword">false</span></span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">/**</span><br><span class="line"> * TestDemo Created by lechance .</span><br><span class="line"> */</span></span><br><span class="line"></span><br><span class="line"><span class="annotation">@MyAnnotation</span>(value = <span class="string">"Non-defaultValue"</span>, isDebugMode = <span class="keyword">true</span>)</span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> <span class="keyword">implements</span> <span class="title">Reflection</span> </span>&#123;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">static</span> Object mSync = <span class="keyword">new</span> Object();</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">static</span> Test Instance = <span class="keyword">new</span> Test();</span><br><span class="line"></span><br><span class="line">    <span class="keyword">static</span> &#123;</span><br><span class="line">        System.err.println(Test.class.hashCode());</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">public</span> String strPubAttribute;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">protected</span> String strProAttribute;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String strPriAttribute;</span><br><span class="line"></span><br><span class="line">    <span class="comment">//the fields from constructor</span></span><br><span class="line">    <span class="keyword">private</span> String description;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> id;</span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> String content;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">Test</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="comment">//default constructor</span></span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">Test</span><span class="params">(String description)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>.description = description;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="title">Test</span><span class="params">(<span class="keyword">int</span> id, String description, String content)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">this</span>(description);</span><br><span class="line">        <span class="keyword">this</span>.id = id;</span><br><span class="line">        <span class="keyword">this</span>.content = content;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>&#123;</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            Test.class.newInstance().foo();</span><br><span class="line">        &#125; <span class="keyword">catch</span> (InstantiationException | IllegalAccessException e) &#123;</span><br><span class="line">            e.printStackTrace();</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">testMethod</span><span class="params">(String input)</span> </span>&#123;</span><br><span class="line">        System.err.println(<span class="string">"this string generated form Reflection mechanism. Argument is: "</span> + input);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> Test <span class="title">getInstance</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="keyword">synchronized</span> (mSync) &#123;</span><br><span class="line">            <span class="keyword">if</span> (Instance == <span class="keyword">null</span>) &#123;</span><br><span class="line">                Instance = <span class="keyword">new</span> Test();</span><br><span class="line">            &#125;</span><br><span class="line">            <span class="keyword">return</span> Instance;</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">foo</span><span class="params">()</span> </span>&#123;</span><br><span class="line">        <span class="class"><span class="keyword">class</span> <span class="title">le</span> </span>&#123;</span><br><span class="line">        &#125;  <span class="comment">//local class</span></span><br><span class="line"></span><br><span class="line">        print(<span class="string">"The name of the class: "</span> + <span class="keyword">this</span>.getClass().getName());</span><br><span class="line">        print(<span class="string">"isAssignableFrom&lt;Test.class&gt;: "</span> + <span class="keyword">this</span>.getClass().isAssignableFrom(Test.class));</span><br><span class="line">        print(<span class="string">"isLocalClass: "</span> + le.class.isLocalClass());</span><br><span class="line">        print(<span class="string">"isInterface: "</span> + Pluginable.class.isInterface());</span><br><span class="line">        print(<span class="string">"CanonicalName: "</span> + <span class="keyword">this</span>.getClass().getCanonicalName());</span><br><span class="line">        print(<span class="string">"getTypeName: "</span> + <span class="keyword">this</span>.getClass().getTypeName());</span><br><span class="line">        print(<span class="keyword">this</span>.getClass().asSubclass(Reflection.class).getSimpleName());</span><br><span class="line">        print(<span class="keyword">this</span>.getClass().getDeclaredAnnotations()[<span class="number">0</span>]);</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            print(<span class="keyword">this</span>.getClass().getDeclaredField(<span class="string">"strPubAttribute"</span>).getType());</span><br><span class="line">        &#125; <span class="keyword">catch</span> (NoSuchFieldException e) &#123;</span><br><span class="line">            e.printStackTrace();</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">try</span> &#123;</span><br><span class="line">            print(<span class="keyword">this</span>.getClass().getMethod(<span class="string">"testMethod"</span>, java.lang.String.class));</span><br><span class="line">            <span class="keyword">this</span>.getClass().getMethod(<span class="string">"testMethod"</span>, java.lang.String.class).invoke(Test.class.newInstance(), <span class="string">"le"</span>);</span><br><span class="line">        &#125; <span class="keyword">catch</span> (NoSuchMethodException | IllegalAccessException | InstantiationException | InvocationTargetException e) &#123;</span><br><span class="line">            e.printStackTrace();</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        print(<span class="string">"---------------getMethods()----Public--Method---------"</span>);</span><br><span class="line">        Method[] methods = <span class="keyword">this</span>.getClass().getMethods();</span><br><span class="line">        <span class="keyword">for</span> (Method method : methods) &#123;</span><br><span class="line">            print(method.getName());</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        print(<span class="string">"---------------getMethods()----declared--Methods---------"</span>);</span><br><span class="line">        Method[] methods1 = <span class="keyword">this</span>.getClass().getDeclaredMethods();</span><br><span class="line">        <span class="keyword">for</span> (Method m : methods1) &#123;</span><br><span class="line">            print(m.getName());</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        Constructor[] constructors = <span class="keyword">this</span>.getClass().getConstructors();</span><br><span class="line">        <span class="keyword">for</span> (Constructor c : constructors) &#123;</span><br><span class="line">           print(<span class="string">"Constructor Name: "</span> + c.getName() + <span class="string">" Modifiers : "</span> + c.getModifiers() + <span class="string">"ParameterCount: "</span> + c.getParameterCount());</span><br><span class="line">        &#125;</span><br><span class="line"></span><br><span class="line">        Method ms = <span class="keyword">this</span>.getClass().getEnclosingMethod();</span><br><span class="line">        print(<span class="string">"getEnclosingMethod: "</span> + ms);</span><br><span class="line">    &#125;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">    <span class="keyword">private</span> &lt;T extends Object&gt; <span class="function"><span class="keyword">void</span> <span class="title">print</span><span class="params">(T t)</span> </span>&#123;</span><br><span class="line">        System.out.println(t);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">AnotherClass</span> </span>&#123;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p><em>Refer to <a href="https://docs.oracle.com/javase/tutorial/reflect/" title="java tutorial" target="_blank" rel="external">Java Tutorial</a></em></p>
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